What is a Knit King Jac 40 Accessory used for it is listed as Brother Knitting Machine Accessory?

The Jac 40 is an accessory for jacqard and knit-in two color work on a standard gauge (4.5 MM) knitting machine. It uses punch cards and patterns over as man as 40 stitches. with a punch card installed in the Jac 40, you activate a lever which moves pins out to match tne card. You then use those pins (holding the Jac 40 behind the working needles of your KM) and push the needles forward then knit one row with two colors in your yarn feeder. Then with a click, you move the Jack 40 card forward and select the next row of pins with which you move the next group of KM needs forward for the second row of two color knitting.

It sounds more complex than it is, once you do it. Instead of moving needles forward manually one at a time for patterns larger than your machine can do (if you don't have an electronic machine), you can use the Jac 40.